Osteocytes comprise over 90% of all bone cells, yet little is known of their function(s) or of the involvement of systemic hormones in regulating their activity. Recent studies support the theory that osteocytes play a major mechanosensory role. How exactly the osteocyte can sense the mechanical forces applied to the bone and then transform this mechanical stimulus into a biological function remains incompletely understood. Several hormones, particularly prostaglandin E2 (PGE2) and parathyroid hormone (PTH) had been implicated in this process. This proposal will investigate the hypothesis that Gsa- signaling in osteocytes is critical for proper mechano-transduction and bone homeostasis. Moreover, our initial data revealed that lack of Gsa-signaling in osteocytes induce hematopoietic abnormalities. The first aim of this proposal will test the hypothesis that Gsa-signaling in osteocytes is important for proper bone acquisition and skeletal homeostasis. The second aim will investigate the hypothesis that Gsa-mediated signaling in osteocyte is an important regulator of mechano-transduction. Lastly, in aim III we will investigate the role of Gsa in hematopoiesis. To address these questions, mice in which Gsa expression is specifically ablated in osteocytes have been generated. To restrict ablation of Gsa to osteocytes, we used the 10Kb upstream promoter of Dentin Matrix Protein-1 (DMP-1), known to be expressed specifically in osteocytes, to drive the Cre-recombinase in mice in which Exon1 of Gnas is flanked by Lox-P sites. This animal model will enable enhanced understanding of Gsa action on bone and on the hematopoietic system and could direct the development of novel therapeutic agents. The studies proposed here will expand the current knowledge on Gsa-signaling in osteocytes and will investigate the extent to which Gsa-dependent pathways in these cells is involved in normal skeletal development, hematopoiesis and mineral-ion homeostasis. In so doing, they promise to significantly expand understanding of osteocyte and bone biology. The hypothesis that we wish to test is that signaling via Gsa in osteocytes directly affects bone homeostasis and mechanosensation and directly, or indirectly controls myeloid cells development and hematopoiesis. If our hypothesis holds true it will be the first time that osteocytes are shown to regulate hematopoiesis.